Rotary machines contain rotor blades installed in rotatable hubs or disks in axial orientation. These rotor assemblies typically contain a plurality of aerodynamically shaped blades, or airfoils, which operate at high speeds and in hostile environments. As a result, the rotor blades are subjected to steady and vibratory loads which generate significant internal and external stresses.
Properly designed rotor blades can accommodate most known loads, resulting in the long and safe operation of the rotary machine. These loads are derived from inertial and aerodynamic sources. Inertial loading depends on rotor blade mass and stiffness distribution, and impacts the vibratory response and frequency tuning of the rotary machine. Aerodynamic loading depends on both steady state and disturbed air flow through the rotary machine. These loads then cause a variety of stresses, such as centrifugal, bending, and torsion, as well as vibratory bending, on the rotary machine. In the case of vibratory bending stresses, it is known that frequency tuning of the rotor blade aids in the control of vibratory response to unsteady loading.
The main source of steady state internal stresses in rotating parts within a rotary machine is centrifugal force. In a rotary machine any typical cross-sectional area of the rotor blade must restrain the centrifugal force acting on all of the material beyond its own radial location in relation to the machine's axis of rotation. It necessarily follows that the largest forces sustained by a rotor blade occur in the blade's root structure. These forces are then transferred through a rotor blade retention apparatus to a centrally-located rotatable hub. In a pinned-root rotor blade these forces are passed from the rotor blade root through retention tenons which are attached to the retention member which is secured to the rotatable hub.
Additional sources of blade stress include foreign object contact, changes in flight altitude and attitude, as well as aircraft-induced shock and vibration. In particular, propulsor blades mounted in an aft position as called for in some aircraft designs are more susceptible to foreign object damage than are normal wing-mounted propulsors. Damage to those aft-mounted blades may be caused by collisions with runway water and slush, ice, sand, stones, and other debris kicked up by the aircraft's tires. Another potentially serious cause of damage to propulsor blades is in-flight bird strikes.
An important problem with rotor blades is accommodation of the aforementioned stresses when the blade is subjected to the operational and adverse stressing conditions described above.
The related art generally teaches the use of a pinned-root engagement to securely capture the blade to the rotor hub, and reduce or relieve root bending stresses. However, pinned-root attachments may experience frictional sliding of blade roots against their retention tenons and retaining pins as installed in a rotatable hub, which may result in undesirable heating, wear, and fretting of the mating surfaces and underlying structures. Also, this type of engagement provides limited accommodation of the external stresses and moments imposed on the blade upon contact with foreign objects. These problems have impeded desired extension of rotor blade service life.
What is needed is a new pinned-root design which can minimize and/or accommodate those forces, moments, and angular excursions of the rotor blade about the pin axis which cause undesirable wear and fretting of a rotor blade in the general vicinity of its connection to a rotatable hub.